15 Feb, 2009

Pros and Cons of Diskless Servers booting off a SAN

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In our constant efforts towards Architecture nirvana we are often faced with the question of whether a cluster of application servers should have their own hard disks or should they PxE boot off a SAN. This short article explores the options

Notes

  • If a cluster of machines have their own OS hard drives, and one cannot afford a machine going down then each of the machines would need a RAID 1 config which requires a RAID card and 2 hard drives each resulting in a considerable cost (high-cost)
  • In the scenario where multiple machines boot off a partition on a SAN device, the machines do not need any harddrives. However if for any reason the connectivity to the SAN goes down or the SAN device itself crashes (rare) then all the machines in the cluster would be down (marginal redundancy concern)

Conclusion

  • In the scenario where the data partition of the cluster of machines is residing on a SAN device, it makes sense to boot those machines off the SAN device too since as such the SAN going down would render the entire cluster useless, and this way one can save the cost of 2x’n’ hard drives and ‘n’ RAID cards (assuming we have ‘n’ machines in the cluster)
  • In the scenario that a cluster of machines does not have any data on a SAN device, one may want to invest in hard drives for the machine itself, since a downtime of the SAN device will not render the cluster inoperational. Additionally, if the cluster consists of 10-15 machines, the cost of 2 SATA drives and 1 RAID card per machine may not be much higher than the cost of a SAN device if one needs to be exclusively setup for these machines.
  • This may change however if one has spare and redundant SAN devices lying around, with spare capacity in their network
  • Ideally if a cluster of machines are to PXE boot off a SAN, one should try and ensure that the boot partitions are spread across separate SAN Devices each of which are accessible through different network paths, so that the downtime of any given SAN Device does not compromise the cluster
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  • Comment by Biju Krishnan on February 28, 2009 @ 9:28 am
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  • Comment by yangyang on March 25, 2010 @ 7:28 am

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Biju Krishnan
February 28, 2009

Another advantage is that you can industrialize server deployment with pre-installed applications.

For example if you are a hosting solutions provider, your new client requires 10 SQL servers to be deployed within a week, if you use advanced SAN technology then you can do this in hours.

if the client wishes to customize base image with his own requirements, then you can offer to store this in such a way that a copy of this is then offered on demand.

Disadvantages

Multipathing still possesses uncertainty inspite of maturity over the years, a human error on a SAN fabric can bring down your entire data center since u depend heavily on it. All shared infra comes with high risks but yes the advantages outweigh risks.

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Sriram Narayanan
June 18, 2009

One could minimize the dangers of a SAN dying by opting for multi-controller SANs configured in continuous replication mode.

The Sun Storage devices have this at present, and the Fishworks UI on them makes it definitely worth the cost (which itself is much lower than comparable SAN appliances from EMC, Netapp and the like).

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yangyang
March 25, 2010

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